feat: container monitoring backend core (PR-1)
- Add instance_events and health_checks tables with Alembic migration - InstanceEventBus: typed pub/sub singleton with wildcard support - HealthMonitor: async background loop polling containers every 15s - SSE endpoint GET /events/stream with auth and connection limits - Lifecycle hooks in tool_instances.py (create/start/stop/restart/delete) - Structured JSON logging with correlation IDs - 15 new unit tests (EventBus, HealthMonitor, MonitoringModels) Quality gates: pytest 15 new passed, ruff clean
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# SDD Proposal: Container Monitoring & Notification System
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## Status
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**Phase:** proposal
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**Date:** 2026-05-28
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**Owner:** Gentle AI
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**Based on:** Exploration `container-monitoring-notifications`
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---
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## 1. Problem Statement
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Users start containers via docker compose, but when something goes wrong — a build error, a missing container, a crashed process, a failed tunnel — there is **zero visibility**. Failures are buried in server logs. The only hint is a generic 4004 error in the terminal or a stale status badge that only updates when the frontend happens to poll (every 30 seconds).
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Current pain points:
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- **Silent failures**: A container exits or a tunnel dies and the user doesn't know until they manually refresh.
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- **No push notifications**: The frontend polls every 30s; status changes have up to 30s latency.
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- **No lifecycle audit trail**: There's no record of when a container started, stopped, or crashed.
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- **No health history**: The `probe_result` JSON blob is overwritten on every restart — no trend data.
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- **Ephemeral logs**: Container stdout/stderr is only available via `docker logs` on-demand; nothing is persisted.
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This gap was surfaced by the terminal feature: when containers fail to build or start, the terminal shows a 4004 error with no explanation, leaving users stuck.
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---
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## 2. Goals
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1. **Real-time status push**: Users see container lifecycle events (start, stop, error, health change) within seconds, not 30s.
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2. **Proactive health monitoring**: Background task continuously monitors running containers and tunnels, not just at startup.
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3. **User notifications**: Toast / alert notifications when containers fail, crash, or become unhealthy.
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4. **Event audit trail**: Append-only log of all instance lifecycle transitions.
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5. **Health history**: Time-series snapshots of container + tunnel health for debugging trends.
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6. **Structured logging**: JSON logs with correlation IDs and instance IDs for traceability.
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---
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## 3. Non-Goals
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- **Auto-restart of crashed containers** (out of scope for MVP; may be added later).
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- **Multi-replica API support** (in-memory event bus is sufficient for now; Redis/NATS deferred).
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- **Log aggregation / shipping to external systems** (e.g., Loki, ELK — structured JSON logs only).
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- **Email / SMS / Slack notifications** (in-app toast only for MVP).
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- **Container resource metrics** (CPU, memory, disk — Docker stats not in scope).
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- **Replacing docker CLI with Docker SDK** (keep existing shell-out pattern).
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---
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## 4. User Stories
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### US-MON-001: Container Start Notification
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> As a user, when I start a container, I want to see a "Container starting..." toast so I know the system is working, followed by a "Container running" toast when it's ready.
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### US-MON-002: Build Failure Alert
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> As a user, when a container fails to build or start, I want an immediate toast with the error message and exit code so I don't have to dig through server logs.
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### US-MON-003: Tunnel Failure Detection
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> As a user, when a Cloudflare tunnel dies while my container is running, I want a real-time notification so I can restart it.
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### US-MON-004: Health Status History
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> As a user, when my container is flapping between healthy and unhealthy, I want to see a history of health checks to diagnose the issue.
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### US-MON-005: Lifecycle Audit
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> As a platform operator, I want an audit log of who started/stopped/restarted which container and when, for troubleshooting and accountability.
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---
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## 5. Proposed Solution
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### Architecture Overview
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```
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┌─────────────────┐ SSE ┌──────────────────┐
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│ Frontend │◄─────────────│ FastAPI │
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│ (toast + │ events │ SSE endpoint │
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│ status badges)│ │ /events/stream │
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└─────────────────┘ └────────┬─────────┘
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│
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┌───────────┴───────────┐
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│ InstanceEventBus │
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│ (in-memory) │
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└───────────┬───────────┘
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│ publish
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┌─────────────────────┼─────────────────────┐
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│ │ │
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┌────────▼────────┐ ┌───────▼────────┐ ┌────────▼────────┐
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│ Lifecycle hooks │ │ Health Monitor │ │ Instance CRUD │
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│ (start/stop/ │ │ (asyncio loop) │ │ (create/delete)│
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│ restart/delete)│ │ │ │ │
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└─────────────────┘ └───────┬────────┘ └─────────────────┘
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│
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┌──────────▼──────────┐
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│ Docker + Tunnel │
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│ (poll every 15s) │
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└─────────────────────┘
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```
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### Components
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#### 5.1 InstanceEventBus (in-memory singleton)
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- Pattern: Same singleton style as `TerminalManager`.
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- Publishes typed events: `instance.created`, `instance.started`, `instance.stopped`, `instance.health_changed`, `instance.error`.
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- Subscribers: SSE endpoint broadcasts to connected clients; health monitor subscribes for its own coordination.
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#### 5.2 Background Health Monitor
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- Pattern: `asyncio` loop, modeled after `TerminalManager._idle_check_loop` (every 60s → every 15s).
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- For each running instance:
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1. Call `docker inspect` for container status + exit code.
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2. For web tools, curl the public URL for tunnel health.
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3. Compare with last known state.
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4. On change: update DB `status`, write `health_checks` row, publish event to bus.
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- On container crash/OOM: publish `instance.error` with exit code and stderr snippet.
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#### 5.3 SSE Endpoint
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```
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GET /events/stream
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```
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- FastAPI `StreamingResponse` with `text/event-stream`.
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- Authenticated (same cookie/JWT as existing API).
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- Sends JSON event payload per line.
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- Frontend reconnects with exponential backoff on disconnect.
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#### 5.4 Frontend Toast Layer
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- New lightweight toast component (e.g., `sonner` or custom).
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- Single SSE connection on app mount.
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- Filters events by relevance (errors always shown; start/stop shown briefly).
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- Also updates instance status badges in real-time (no more 30s polling lag).
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#### 5.5 Database Additions
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**New table: `instance_events`** — append-only audit log
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```
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id UUID PK
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instance_id UUID FK → tool_instances.id ON DELETE CASCADE
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event_type VARCHAR(50) -- created, started, stopped, restarted, deleted, health_changed, error
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status VARCHAR(50) -- snapshot of instance status at time of event
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message TEXT -- human-readable description / error message
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created_by UUID FK → users.id
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metadata JSONB -- exit_code, probe_output, tunnel_url, etc.
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created_at TIMESTAMPTZ DEFAULT now()
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```
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**New table: `health_checks`** — periodic health snapshots
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```
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id UUID PK
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instance_id UUID FK → tool_instances.id ON DELETE CASCADE
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container_status VARCHAR(50) -- running, exited, dead, etc.
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container_healthy BOOLEAN
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tunnel_healthy BOOLEAN
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exit_code INT
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probe_status VARCHAR(50)
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probe_output TEXT
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checked_at TIMESTAMPTZ DEFAULT now()
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```
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#### 5.6 Structured Logging
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- Switch API container logs to JSON format.
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- Fields: `timestamp`, `level`, `logger`, `message`, `instance_id`, `event_type`, `correlation_id`.
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- Container stdout/stderr remains in Docker; we do not duplicate it.
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---
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## 6. Key Decisions
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| Decision | Choice | Rationale |
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|----------|--------|-----------|
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| **Event transport** | **SSE** (not WebSocket) | One-way server→client push is all we need. SSE is simpler, uses HTTP, works through proxies, and FastAPI supports it natively. WebSocket is overkill and only used for terminal bidirectional streams. |
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| **Event bus** | **In-memory** (not Redis/NATS) | No new infrastructure. Single API process assumption holds today. TerminalManager already uses in-memory state. Defer distributed bus to when horizontal scaling is needed. |
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| **Health monitoring** | **Background asyncio poll** (not Docker events API) | Docker CLI events API requires a persistent stream and is tricky with shell-outs. A simple poll loop every 15s is predictable, testable, and matches our existing `docker inspect` usage. |
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| **Frontend polling** | **Eliminate for status** (keep for list refresh) | Instance list may still poll occasionally, but status changes and errors push via SSE. Reduces server load and gives instant UX. |
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| **Notification scope** | **In-app toast only** | No external integrations for MVP. Keeps scope tight. Toast library (e.g., `sonner`) is a small dependency. |
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| **Log persistence** | **Structured JSON to stdout only** | We do not build a log storage system. Docker already retains container logs. Our structured API logs can be shipped later if needed. |
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| **Auto-restart** | **Out of scope** | Detect and notify, but do not automatically restart crashed containers. User must explicitly restart to avoid surprise side effects. |
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---
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## 7. Risks
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| Risk | Severity | Likelihood | Mitigation |
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|------|----------|------------|------------|
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| **Docker CLI brittleness under load** | Medium | Medium | Keep poll interval conservative (15s). Reuse existing `docker.py` service; do not add new CLI patterns. Monitor `execute_compose_command` latency. |
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| **SSE connection leaks** | Medium | Low | Use FastAPI background task cleanup. Close stream on client disconnect. Limit max connections per user (e.g., 5). |
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| **Memory growth from event bus** | Low | Low | Event bus holds only subscriber references, not event history. Health monitor does not retain old check results. |
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| **Tunnel PID fragility** | High | High | Existing risk, not introduced by this change. Health monitor will at least *detect* leaked/orphaned tunnels and surface them. |
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| **Frontend SSE reconnect storms** | Medium | Low | Exponential backoff on reconnect. Jitter to prevent thundering herd. |
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| **Database write amplification** | Medium | Medium | Health checks every 15s × N running instances. Write only on state change, not every poll. `health_checks` table may grow; add retention policy (e.g., 30 days) in follow-up. |
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| **Scope creep into full observability** | High | Medium | Explicitly exclude metrics dashboards, log storage, alerting rules, and PagerDuty-style on-call. Stay focused on lifecycle events + toast. |
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| **Multi-replica incompatibility** | Low | Low | Document that in-memory bus won't work across replicas. Add Redis/NATS only when scaling need is proven. |
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---
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## 8. Acceptance Criteria
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- [ ] **AC-1:** `POST /instances/{id}/start` publishes `instance.started` event; frontend shows "Container starting..." toast.
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- [ ] **AC-2:** If container fails during start (exit code ≠ 0), `instance.error` event is published within 5s; frontend shows error toast with message + exit code.
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- [ ] **AC-3:** Background health monitor runs every 15s and detects container crashes, OOMs, and tunnel failures.
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- [ ] **AC-4:** On health state change (e.g., `running → unhealthy`), `instance.health_changed` event pushes via SSE and updates status badge without page refresh.
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- [ ] **AC-5:** `instance_events` table records every lifecycle transition with `event_type`, `status`, `message`, and `created_by`.
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- [ ] **AC-6:** `health_checks` table records a row on every state change (not every poll) with `container_status`, `tunnel_healthy`, `exit_code`, `checked_at`.
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- [ ] **AC-7:** Frontend establishes one SSE connection on app load and receives events for all user's instances.
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- [ ] **AC-8:** API logs are emitted in JSON format with `instance_id`, `event_type`, and `correlation_id` fields.
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- [ ] **AC-9:** No regression in existing terminal WebSocket, instance CRUD, or tunnel functionality.
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- [ ] **AC-10:** Backend tests cover event bus publish/subscribe, health monitor state transitions, and SSE endpoint auth.
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---
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## Effort Estimate
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| Phase | Files | Lines (est) | Complexity |
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|-------|-------|-------------|------------|
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| DB migrations + models (`instance_events`, `health_checks`) | 3 | 150 | Low |
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| InstanceEventBus backend | 2 | 200 | Low |
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| Health monitor background task | 2 | 300 | Medium |
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| SSE endpoint + auth | 2 | 200 | Medium |
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| Lifecycle hook instrumentation | 3 | 200 | Low |
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| Frontend toast component + SSE client | 4 | 400 | Medium |
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| Real-time status badge updates | 3 | 150 | Low |
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| Structured logging refactor | 2 | 100 | Low |
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| Tests | 4 | 400 | Medium |
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| **Total** | **25** | **~2100** | **Medium** |
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**Review workload forecast:** ~2100 lines exceeds the 400-line budget. Recommend **chained PRs**:
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1. **Backend core**: Event bus, health monitor, DB migrations, SSE endpoint (~1000 lines)
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2. **Frontend**: Toast component, SSE client, real-time badge updates (~700 lines)
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3. **Integration + logging**: Structured JSON logs, lifecycle hooks, tests (~400 lines)
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---
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## Next Recommended Phase
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**Design** — Detail the `InstanceEventBus` interface, health monitor state machine, SSE payload schema, and toast UX behavior. Then proceed to `tasks.md` for implementation breakdown.
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